🎨 Palette: Added loading states to form submit buttons

[singhaditya21]

💡 What

Added async loading states to the Login and Register form submit buttons in the vanilla JS web demo. When a user submits either form, the button is immediately disabled and its text changes to indicate progress (e.g., "Logging in..." or "Creating account...").

🎯 Why

To prevent users from repeatedly clicking the submit button during API delays, and to provide immediate visual feedback that their action is being processed. This solves a common UX issue where slow network requests can make an app feel unresponsive or lead to duplicate form submissions.

📸 Before/After

Before: Clicking submit showed a toast notification, but the button itself remained active, clickable, and visually unchanged.
After: Clicking submit disables the button (lowered opacity, not-allowed cursor) and updates its text to reflect the loading state until the request finishes.

♿ Accessibility

This enhancement improves accessibility by ensuring the triggering element (e.submitter) explicitly enters a disabled state during async operations, which screen readers can announce. It also relies on a robust finally block to restore the button's interactivity and content whether the API request succeeds or fails, preventing users from getting stuck in an inescapable state.

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PR created automatically by Jules for task 4740889567211729502 started by @singhaditya21

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